When I asked my wife why the sky is dark at night, she said, “It’s because the sun’s not shining!” Yet generations of astronomers have struggled to answer this question. It’s become known as Olbers’ Paradox. Although it’s one that few spend time thinking about, for cosmologists it’s always been important.
The darkness of the night sky was one of the earliest observations that cosmologists could use as a basis for developing an understanding of the universe. In theory, they knew that even at night the sky should shine like a curtain of light. Explaining why it didn’t helped cosmology to be seen as a true science that is based on observation rather than speculation. It was one of the first observations that helped us realize that the universe is not infinite either in age or extent.
To light the sky from horizon to horizon with starlight would require trillions more stars than now exist. Alternatively, we’d have to be able to look back in time longer than the universe has existed. The universe is not old enough or big enough to have a bright night sky. As we look beyond the galaxies and quasars, we detect only the background cosmic radiation left as a remnant from the moment of creation, when the sky really did blaze.
So next time you gaze at the night sky, remember you’re seeing a puzzle, whose explanation has helped us understand the universe.